The present invention relates generally to gas turbine engines, and, more specifically, to film cooling therein.
In a gas turbine engine air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. Energy is extracted from the gases in a high pressure turbine (HPT) which powers the compressor, and in a low pressure turbine (LPT) which powers a fan in a turbofan aircraft engine application, or powers an external shaft for marine and industrial applications.
Engine efficiency increases with temperature of combustion gases, but the combustion gases heat the various components along their flowpath, which in turn requires cooling thereof for obtaining a long life of the engine. The flowpath components subject to the hot combustion gases are cooled by bleeding air from the compressor which correspondingly reduces engine efficiency since the bled air is not used in the combustion process.
Accordingly, gas turbine engine cooling art is mature and includes innumerable patents for minute differences in cooling circuits and features in the various components of the hot flowpath.
For example, the combustor includes radially outer and inner liners which require cooling during operation. Turbine nozzles include hollow vanes supported between outer aid inner bands which also require cooling. Turbine rotor blades are hollow and typically include cooling circuits therein, with the blades being surrounded by turbine shrouds which also require cooling. The hot combustion gases are discharged through an exhaust which may also be lined, and suitably cooled.
In all of these exemplary gas turbine engine components, thin metal walls of high strength superalloy metals are typically used for enhanced durability while minimizing the need for cooling thereof. Various cooling circuits and features are tailored for these individual components in their corresponding environments in the engine, but all these components typically include common rows of film cooling holes.
A typical film cooling hole is a cylindrical bore inclined at a shallow angle through the heated wall for discharging a film of cooling air along the external surface of the wall to provide thermal insulation against the hot combustion gases which flow thereover during operation. The film is discharged at a shallow angle over the wall outer surface to minimize the likelihood of undesirable blow-off thereof which would lead to flow separation and a loss of the film cooling effectiveness.
Furthermore, the film cooling holes are typically arranged in rows of closely spaced apart holes which collectively provide a large area cooling air blanket over the external surface.
However, the more holes required for providing full-surface coverage of the film cooling boundary layer, the more air is also required which therefore decreases engine efficiency.
Accordingly, the art of film cooling holes itself is replete with numerous patents on the minute details of various forms of those holes for improving their efficiency for maintaining flow attachment with the widest possible lateral dispersion. For example, the discharge end of the simple cylindrical film cooling bore may diverge to the outer surface of the wall being cooled for diffusing the pressurized cooling air as it is discharged from the outlet. Diffusion is an aerodynamic mechanism in which the high velocity of the pressurized cooling air is reduced which in turn increases its pressure.
Diffusion outlets for typical film cooling holes decrease the discharge velocity thereof for ensuring good flow attachment or the discharged film cooling air without undesirable flow separation. And, the lateral width of the diffusion outlet correspondingly increases the lateral flow coverage of the film.
However, diffusion in film cooling holes has a typical limit due to the diverging half-angle of the diffusion outlet to prevent undesirable flow separation. For example, the diffusion angle is typically limited to about ten degrees on each side of the diffusion outlet to prevent overexpansion of the discharge cooling air which could lead to undesirable film separation.
Accordingly, it is desired to provide an improved film cooling hole having increased film coverage while maintaining good flow attachment of the discharged film cooling air.